1. Field of the Invention
The present invention relates to mixing of ingredients in closed containers, which may be rigid or flexible, such as bags. Typical applications are pharmaceutical and biological manufacturing, and involve the dissolution of solids, reconstitution of biological media, and mixing of sterile suspensions.
2. Description of the Related Art
In various industries, especially pharmaceuticals, many materials are stored in disposable plastic bottles and bags. These one-use containers are very cost effective because they do not require to be cleaned and sterilized prior to and after use. Such bags and bottles are used to store dry ingredients prior to reconstitution, such as components for buffers and liquids such as culture media; or solutions, such as intermediate products prior to further processing.
The major limitation to the increased use of such containers is the inability to mix the ingredients contained in the bag. This is especially serious with large bags (capacities of 10 to 1000 liters) which cannot be shaken by hand. Current art requires that the contents of the bag be transferred to a mixing tank and the ingredients mixed by a conventional paddle or impeller-type mixer. After mixing, the ingredients need to be transferred back into a bag for storage. This method has several drawbacks—1) the need for an expensive rigid mixing tank and mixer that must be cleaned before and after use; 2) the need for a second disposable container for the material after mixing; 3) difficulty in maintaining sterility during this operation; and 4) significant labor-intensive fluid transfers.
Attempts have been made to mix ingredients inside a bag. One method is to provide a dip tube and to use an external pump to pump the contents of the bag through a tubing loop back into the bag (U.S. Pat. No. 5,362,642). This method is of very limited effectiveness. Firstly, materials tend to sediment in the corners of the bag where the dip tube cannot reach, so that they are never dispersed. Secondly, for effective mixing a high pump-around flow rate is required. In a non-rigid container, such as a bag, suction develops near the intake of the dip tube due to the high flowrate. This causes the wall of the bag to collapse, choking off the flow in the pump-around loop and decreasing the mixing efficiency. Another method that has been reported, is the insertion of a magnetic stirrer assembly into the bag prior to fill. The bag is then positioned on a motorized drive assembly that forces the magnetic stir bar inside the bag to rotate. This technique has the advantage that it provides a non-invasive means of agitating the contents of the bag. However, a simple calculation of power input and fluid properties will show that this method cannot impart sufficient energy to mix a bag larger than say 5 liters in volume within a reasonable period of time. Thus, it is useless for the majority of mixing applications that involve mixing 10 to 1000 liters of liquid in a bag.
A common method for non-invasively mixing viscous fluids, such as paint, is the use of high frequency shakers. Examples are Micin (U.S. Pat. No. 3,788,611), Powell (U.S. Pat. No. 4,662,760) and Lorenzen (U.S. Pat. No. 3,735,964). These devices are designed to be operated at 1000 or more cycles per second and angles of oscillation of 20 to 120 degrees. They are restricted to small volumes (less than 4 liters) since the cost of mechanisms necessary to handle the inertia and momentum of greater masses is prohibitive. The mixing conditions cited in these shaker patents are far too harsh for biological fluids. It is also doubtful that a flexible bag could be made that would withstand this high speed shaking. Thus, such shaking devices are of little use in developing a method for mixing large volumes (5 to 1000 liters) of biological fluids.
Another technique uses a kneading motion to mix inside the bag. U.S. Pat. Nos. 3,297,152, 3,819,107, 4,557,377 and 5,779,974 have specialized bag designs, some with multiple internal pockets, that are used to mix specific components. Examples are epoxy resins and food ingredients. While these methods are quite efficient, they are not useful for general purpose use nor can they be scaled up to the large volumes necessary. These methods are not usable with standard storage bags, which consist of a single chamber of “pillow” or “cube” construction with single inlet and outlet ports.
The idea of using a rocking motion to mix liquids is not new. U.S. Pat. No. 1,937,422 employs a rocking platform to mix photographic solutions in a tray. U.S. Pat. No. 4,146,364 utilizes the concept to mix liquid in test tubes. The obvious extension of this rocking idea is to use a bag or similar flexible container to contain the liquid to be mixed and then place the bag inside a rocking tray. Numerous U.S. patents (U.S. Pat. Nos. 3,583,400, 3,698,494, 3,924,700, and 5,680,110) have been granted for this idea which is quite successful for small bags (less than 500 ml) that are commonly used for blood collection. Another application is a rocking apparatus for cell culture (U.S. Pat. No. 5,071,760). U.S. Pat. No. 3,788,611 has a variant of this idea for small flasks.
U.S. Pat. No. 4,784,297 discloses a beverage mixer based on rocking a filled flexible bag. While this is apparently successful, the patent requires the rocking motion to be in excess of 100 revolutions per minute. Practical experience and numerous citations demonstrate that with a biological solution, such as culture media, such a high agitation rate would cause foaming and rapidly degrade any proteinous components. The reason for this poor mixing efficiency is the lack of gas-filled headspace in the mixing bag of Katz. While others (U.S. Pat. No. 4,470,703) have recognized the importance of free headspace in a mixing bag, this was not foreseen nor was it obvious to Katz.
Very little prior art describes a mixing method or apparatus for large mixing bags (volume greater than 5 liters). Most are limited to blood bags (100 to 1000 ml). Some examples for larger bags are Garlinghouse (U.S. Pat. No. 3,132,848) and Nickerson (U.S. Pat. No. 3,860,219). These applications are for mixing viscous materials such as cement slurry. The method employed by Nickerson envisions essentially rolling the mixing through 180 degrees or even tumbling through 360 degrees. These techniques are not suitable for low viscosity (1 to 20 cP) fluids typical of biological applications. Garlinghouse proposes a wide variety of rocking and mixing mechanisms. Some limitations are worthy of note as they will become apparent when considering the present invention. Firstly, the rocking tilt angle required for mixing is determined by Garlinghouse to be between 5 and 30 degrees. The second item of note is that Garlinghouse specifically restricts the operation of the device to where the generated wave “ . . . is not one which is productive of a resonant effect . . . .”
One method for mixing large volumes of fluid is by Singh (U.S. Pat. No. 6,190,913). However, the primary purpose of this method is for cell culture where aeration is the main objective. The rocking motion necessary for mixing utilizing this method requires a high rocking rate (10 to 30 revolutions per minutes) and relatively high angle (5 to 10 degrees) of tilt. These conditions result in an expensive bag necessary to withstand the high stresses resulting from the high rocking rate and angle. The energy consumption to achieve mixing is also quite large, making this method not very desirable for mixing applications, especially for large volumes.
Accordingly, there is a need for a method for mixing ingredients within a standard storage bag using a non-invasive apparatus that can handle volumes up to 1000 liters. For the method to be successful it must therefore:    be able to utilize bags of standard design and construction;    be able to handle bag volumes up to 1000 liters without leakage;            provide sufficient mixing to provide a homogeneous environment and disperse components in a reasonably short (few minutes) processing time;            maintain a sterile and closed environment in the bag; and    be low cost by reducing mechanical and instrumentation complexity to a minimum.
The present invention will provide a new and improved method for mixing ingredients in a bag that achieves all these criteria and overcomes all the aforementioned prior art limitations.